1. Field of the Invention
The present invention relates to a process for the gas-phase hydrogenation of cyclododecatriene and/or cyclododecadiene to cyclododecene over a solid catalyst comprising a catalytically active metal of group VIII of the Periodic Table of the Elements.
2. Description of the Invention
The selective hydrogenation of cyclododecatriene to cyclododecene has frequently been described in the literature, and numerous attempts have been made to carry out selective hydrogenation with a high degree of conversion and high selectivity.
According to prevailing opinion, the hydrogenation of cyclododecatriene (hereinafter referred to as CDT), which is predominantly in the form of cis,trans,trans-1,5,9-cyclododecatriene, occurs stepwise via dienes, predominantly trans,trans-1,5-cyclododecadiene and cis,trans-1,5-cyclododecadiene (both hereinafter referred to as CDD) and the isomer mixture of trans- and cis-cyclododecene (hereinafter referred to as CDEN) corresponding virtually to the equilibrium to cyclododecane (hereinafter referred to as CDAN).
Since doubly unsaturated compounds such as CDD lead to formation of relatively high-boiling fractions in subsequent reactions such as hydroformylations or hydrations, the degree of conversion of CDD and CDT to CDEN is desirably greater than 99.0%. The selectivity of CDEN formation relative to all products should be above 90% so that the space-time yield in subsequent reactions of CDEN is not reduced by the amount of inert CDAN formed.
One possible way of achieving a high yield of CDEN is homogeneous selective hydrogenation using Ru complexes. U.S. Pat. No. 5,180,870 describes the homogeneous hydrogenation of CDT using Ru complexes. Here, sterically hindered amines and free triarylphosphine are added in order to partially poison the catalyst. Separation of the CDEN from the catalyst and the amines is difficult and leads to potential contamination of the CDEN product with amines. In the patents U.S. Pat. No. 5,177,278, U.S. Pat. No. 3,804,914, U.S. Pat. No. 5,210,349, and U.S. Pat. No. 5,128,296, solvents are added to the homogeneous hydrogenation of CDT with Ru complexes. This leads to low space-time yields and problems in the separation of catalyst and solvent from the reaction product. U.S. Pat. No. 6,194,624 describes homogeneous hydrogenation likewise using a ruthenium catalyst with addition of carbon monoxide and carboxylic acids such as acetic acid or propionic acid. Here too, the separation of the CDEN from the homogeneous catalyst and the carboxylic acid is problematical.
Processes for the heterogeneously catalyzed preparation of CDEN have also been described in the literature. Palladium catalysts are particularly suitable for this heterogeneous catalysis.
In addition, studies using activated copper on an oxidic support have also been reported. The results obtained using copper catalysts which have been presented by Castro et al. in Stud. Surf. Sci. Catal. (1991), 63 (Prep. Catal. V), 95–102 show a notable selectivity to CDEN but only a very low activity. Furthermore, both the selectivity and the activity are dependent to a considerable degree on the preparation and quality of the catalyst system. Industrial use of these catalysts is therefore very difficult since reproducibility of the results is not guaranteed.
The heterogeneous preparation of CDEN may be carried out by, for example, batchwise hydrogenation in the liquid phase using a suspended catalyst, continuous three-phase hydrogenation in a fixed bed (CDT in the liquid phase) and continuous gas-phase hydrogenation over a fixed bed (CDT in the gas phase).
U.S. Pat. No. 3,400,164 and U.S. Pat. No. 3,400,166 and GB 19680712 describe a batchwise hydrogenation in the liquid phase using a suspended catalyst and show that a CDEN selectivity of about 94% can be achieved at a high CDT/CDD conversion when using a palladium catalyst on a support (5% Pd on activated carbon). 1.4 g of this catalyst are used per 100 g of CDT. The reaction is carried out at 160° C. The hydrogen pressure is maintained at 2.07 bar until 75% of the total hydrogen used has been consumed, then at 0.69 bar to a hydrogen consumption of 90% and finally at 0.345 bar. The hydrogenation is then complete after about one hour. Based on the mass of palladium used, the average throughput is 71 g (CDT)/g (Pd)·h.
A disadvantage of this slurry variant of a batch process is the removal of the catalyst, particularly when it has been abraded after some time. A subsequent filtration step complicates the process and inevitably leads to losses in yield.
In addition, the patents U.S. Pat. No. 3,400,164 and U.S. Pat. No. 3,400,166 indicate that aromatic cyclic compounds are formed to an increased extent as by-products at elevated temperatures above 160° C. The patent documents U.S. Pat. No. 3,400,165 and DE 1 678 829 state that these aromatic compounds are formed from CDT. These aromatic by-products are difficult to separate from the desired product and require an increased outlay for purification. For this reason, a process is carried out at 160° C. to provide a very high conversion of CDT and the further conversion of the resulting CDD is carried out at a significantly higher temperature. These aromatic by-products are difficult to separate off and require an increased outlay for purification.
Furthermore, batch processes are generally not suitable for large-scale industrial production and only acceptable when there is no economically viable alternative. Adopting a hydrogenation using a suspended catalyst into a continuous process would raise considerable difficulties and would also be complicated since the characteristics of a plug flow reactor are desirable for subsequent reactions and formation of the intermediate target product. This would require a reactor cascade having a relatively large number of stirred vessels. After each stirred vessel, the catalyst would have to be separated from the outflowing product and returned to the stirred vessel. Such a procedure is therefore difficult to carry out on an industrial scale.
Alternatives may include a trickle-bed reactor in which CDT in liquid form is passed over a fixed bed of palladium-coated pellets in the presence of hydrogen and a fixed-bed process where the liquid and gas are introduced from the bottom. Experiments using such reactors/procedures are described in Catal. Today (F. Stueber et al., 1995, 24(1–2), 95–101) and in Chemical Engineering Science (R. V. Chaudhari, 2001, 65(2), 557–564). In the experiments described in Catal. Today, a coated catalyst having a palladium-containing coating thickness of 240 μm and a hydrogen pressure of 1.5–12 bar were employed. When the reactants were introduced from the bottom, a CDEN yield of only 70% at a (CDT+CDD) conversion of 85% was achieved. It was found in this case that activity problems arise because of diffusion and transport phenomena which are due to the choice of a three-phase system of solid/liquid/gas. The authors identified liquid/solid mass transfer as the limiting factor.
Furthermore, the experiments using a fixed-bed or trickle-bed catalyst displayed a low selectivity with regard to the simultaneous formation of CDAN, as indicated by the significant formation of CDAN occuring at the maximum CDEN formation.
In addition, the downflow and upflow modes are compared in the article in Chemical Engineering Science, with a somewhat higher selectivity being found in the case of the latter. The hydrogen pressure was 12 bar. The yields of CDEN reported were not above 35%. The low yields show that industrial implementation of this process may not be viable.
Gas-phase hydrogenation of cyclododecatriene is known from Wieβmeier (Ind. Eng. Chem. Res. (1996), 35(12), 4412–4416) where a specially produced monolithic microstructure reactor having microchannels and uniform mesopore systems is employed. This reactor is produced by generating a uniform oxidic layer having a thickness of 20–50 μm, depending on the electrolysis time, on structured aluminum wire by anodic oxidation. This layer has pores of uniform length, uniform diameter and uniform spacing aligned perpendicular to the surface in a hexagonal pattern (i.e. the layer thickness is constant). The pores are closed at one end at the oxide/aluminum interface. The pores are repeatedly impregnated with the solution of a Pd compound, dried, calcined and finally reduced so that these pores become uniformly coated with Pd crystallites (degree of dispersion about 0.2–0.3).
Use of this reactor makes it possible to achieve a conversion of more than 96% and a selectivity to cyclododecene of 88%. At the same time, 8% of CDAN was formed and the throughput per amount of catalyst was 36 g (CDT)/g(Pd)·h.
This reactor system is disadvantaged in that the production of the catalyst is very complicated and such a system is therefore unsuitable for industrial production of cyclododecene.
Furthermore, this article states that commercial catalysts comprising a support material with very fine noble metal particles dispersed therein cannot be used for the gas-phase hydrogenation of cyclododecatriene. The nonuniform distribution of the pores on the surface of these conventional supports and the associated nonuniformity of the active metal species on the surface increase the mass transfer problems which are primarily responsible for the poor activity of the catalyst in the article. The authors rule out the industrial use of commercial catalysts.